The invention relates generally to draw out switchgear for electric power systems, and more particularly to drawout switchgear for use with underground cable distribution systems.
Various safety codes and operating procedures for underground power systems require a visible break disconnect be disposed between submersible power distribution equipment, such as switchgear or power transformers, and the shielded underground feeder cables to this equipment. Provisions for performing routine maintainence work on the cable system, such as line energization checks, grounding, fault location, or hi-potting, may also be required. To comply with these requirements, switchgear apparatus for underground power systems generally are equipped with standardized source and load terminal bushings which, when connected with respective matching cable connectors that include provisions for performing the above mentioned routine maintenance work, form fully shielded, waterproof, current exchange assemblies between the feeder cables and the switch-gear bushing terminals. These standardized cable connectors are commercially available for various standard voltage ratings up to 23Kv phase-to-ground, at a continuous current rating of either 200 or 600 amperes. These connectors may be either load-break type connectors, such as that described in U.S. Pat. No. 3,930,709 issued Jan. 6, 1976 to Stanger et al, or non-load break type connectors, which are designed to be connected or disconnected by one man with a standard hot line tool.
Typical underground shielded cable consists of many strands of wire (copper or aluminum) which is first wrapped with a semi-conducting shielding material. Next, a layer of insulation (usually a polyethylene material) is added, followed by an outside layer of semi-conducting shield material. Finally, the neutral wires are spirally wound about this outside layer. Depending on the voltage class, submersible cable may be in the order of two inches in diameter and, because of its construction described above, is relatively inflexible and has a large minimum bending radius. As a result, the space requirements for movement of the cable connector assembly connecting feeder cables to underground switchgear is considerable. For a three phase switchgear device, six cables and cable connectors are required, which results in a large space requirement solely for these cable connectors.
Generally the underground switchgear bushing has an axially threaded terminal to which the cable connector is attached by bolting thereto. Because of the size and stiffness of 600 ampere cables and cable connectors and this bolt-nut method of attachment, as well as the fact that each feeder cable must be individually connected or disconnected, considerable time is required to disconnect or reconnect underground switchgear equipment.